ROME — The comic who is leading a potent new political force in Italy is ruling out joining any alliance to form a new government, dashing hopes of breaking the country’s postelection stalemate.

Beppe Grillo, whose stunning success in this week’s vote makes his 5 Star Movement the third-biggest bloc in Parliament, wrote on his blog Wednesday that his party will not back a confidence vote on any new government formed by mainstream parties.

Grillo instead wants a new election soon. His protest party had railed at the failings of Italy’s political class.

The political uncertainty is roiling financial markets worldwide. So far the top vote-getter, center-left leader Pier Luigi Bersani, is resisting offers by conservative Silvio Berlusconi of a “grand coalition” government that would include Berlusconi’s second-place party.